1. Field of the Invention
The present invention relates to superconducting wire using oxide superconductors, especially, relates to superconducting coils, superconducting current leads, and superconducting cables having preferable degree of uniformity in magnetic field and stability, and superconducting apparatus using same.
2. Description of the Prior Art
Oxide superconductors have been extensively investigated since the discovery of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. (M. K. Wu, J. R. Ashburn, C. T. Torng, Y. Q. Wand and C. W. Chu: Phys. Rev. Letter, 58 (1987) 908) having a critical temperature higher than a liquid nitrogen temperature with the discovery of (La/Ba).sub.2 Cu.sub.1 O.sub.4-.delta. as a starting point, because realization of superconductor applied apparatus operable at the liquid nitrogen temperature can be expected.
Especially, development of materials having higher critical temperature than previous one is remarkable, and it has been found that Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8-.delta. .multidot.Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-.delta. are superconductors having a critical temperature of about 80K and about 105K, respectively, (H. Maeda, Y. Tanaka, M. Fukutomi, and T. Asano: Jpn. J. Appl. Phys., 27, (1988) L209), Tl.sub.2 Ba.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8-67 .multidot.Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-.delta. are superconductors having a critical temperature of about 110K and about 125K, respectively, (Z. Z. Sheng and A. M. Hermann: Nature 332 (1988) 55), and Tl.sub.1 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.7-.delta..multidot.Tl.sub.1 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9-.delta. are superconductors having a critical temperature of about 75K and about 100K, respectively, (S. Matsuda, S. Takeuchi, A. Soeta, T. Suzuki, T. Kamo: Jpn. J. Appl. Phys., 27 (1988) 2062). Each of all the above superconductors has a superconducting phase having a critical temperature higher than a liquid nitrogen temperature, and since then, material development for improving characteristics of the materials have been continued in order to obtain materials having a high critical temperature, high chemical stability, and a single phase obtainable easily by synthesizing derivatives of which a part of composition is replaced with various elements.
As for application technology operating at the liquid nitrogen temperature for utilizing most effectively feature of the above described high temperature superconducting materials, a conductor such as cables and coils used in a condition of electric resistance zero by making the materials a wire, and usages for thin film superconducting devices have been proposed. Especially, for application to energy related fields, development of oxide superconducting wire is an important technical point. Regarding to technology for manufacturing wires from the oxide superconducting materials, there are many methods are proposed such as a wire draw-rolling method wherein metal such as silver is used as sheath material, a jelly-rolling method wherein oxide superconducting material is applied or laminated on a silver plate, scrolled in a concentric manner, and then the roll is drawn, and a method wherein oxide superconducting material is applied on a silver tape, and so on. In the above examples, gold, silver, or their alloy materials are used as the sheath material or base plate material in view of electrochemical equilibrium because the oxide superconducting material becomes partly a molten condition in a heat treatment process, shows characteristics of a caustic molten salt, and corrodes ordinary metallic conductor material such as copper-iron group.
When manufacturing coils from wire obtained by the above described method, there are some technical problems. When superconducting wire is wound in a coil shape, it is necessary to apply a predetermined tension to the wire in order to maintain stability of the coil. Further, wound wire is effected strongly by electromagnetic force as a hoop stress which is caused by magnetic field and current generated by operation of the wound wire as a magnet. Therefore, a specified tensile strength is required. However, gold and silver which are used as materials for sheaths and base plates at present are materials having a low rigidity, and accordingly, the materials do not satisfy the above strength required for winding or operation.
Further, when a composite oxide superconductor using the sheath material and the base plate is used for permanent switches, current leads, and cables of a superconducting coil or an apparatus thereof using metallic group superconductor and liquid helium as a coolant, there are some cases wherein thermal conductivities of the sheath material and the base plate material must be decreased, or electric resistances of them must be increased. In the above described cases, if the sheath material and the base plate material are gold or silver, the materials are disadvantageous in practical use.
When thinking about an application of the above described oxide superconductors to a coil, a permanent switch, a current lead, a cable, and others by manufacturing wires, gold or silver is selected as materials for sheaths and base plates in view of a chemical equilibrium. However, gold and silver do not satisfy requirements for a practical material of wire in characteristics such as a preferable mechanical strength of the conductor, a low thermal conductivity, and a wide design selection range in electric resistance under a normal conducting state. That is, in order to realize a compact superconducting coil having an uniform magnetic field and a preferable stability, such wire is required as the one which has a characteristics as of a stabilized material with sufficiently low electric resistance as for sheath material or base plate material, and a sufficient rigidity for enduring electromagnetic force at winding the wire for forming a coil or at operation. Further, in order to realize a mechanically strong current lead having a small heat absorbing property, such sheath material or base plate material is required as the one which has a small thermal conductivity and a preferable mechanical strength. Furthermore, in order to realize a permanent current switch made of superconductor, sheath material or base plate material having a preferable high electric resistance is required.